The present invention relates to a process for preparing highly disperse silicon dioxide by combustion of organochlorosilicon compounds.
DE-26 20 737 C2 describes a process by which highly disperse silicon dioxide of high chemical purity can be prepared by combustion of organochlorosilane compounds in the presence of atmospheric oxygen and a burnable gas. The organochlorosilanes used are present in the form of a vaporizable liquid which consists of one or more of the pure organosilane compounds. Use of liquids which, in addition to an organochlorosilane compound, also contain hydrocarbons, is not described in the patent. A use of this type is also not obvious, since according to the patent "with organosilanes, generally, only dark, carbon-contaminated products are achieved". For the combustion process, the liquid is vaporized and the vapor is mixed with the remaining gases which participate in the combustion.
According to DE-A-29 09 815, it is also possible to burn gaseous or vaporizable silicon compounds, which could not be supplied to any economical use, to give highly disperse silicon dioxide. As an example, hydrocarbon-containing by-products are mentioned which arise as first runnings and tailings in the separation by distillation of the products from the reaction of silicon or silicon alloys with organic halides or hydrogen chloride. The addition of a combustible gas is omitted in the combustion and, instead, steam is fed to the gas mixture intended for combustion. However, it has turned out that this process is scarcely suitable for industrial use, because some of the organochlorosilane prematurely reacts with steam to form hydrolyzed silicic acid, which in a short time plugs the feed tube to the combustion chamber and its removal is associated with considerable safety problems.